What happens if you bring a wild animal into captivity? Because of
its "hardy" constitution, is it actually more fit than its
"degenerate" domestic cousins? What happens to wild animals that are
kept for generations in captivity?

What about the reverse -- releasing a domestic animal in the wild?
Is it helpless, a soft urbanite released into the harsh natural
world? Does it revert to its wild roots?

These two situations are actually very similar. In both cases, an
animal adapted to a particular environment is placed in a drastically
different environment. The criteria for success are dramatically
changed. In the wild environment, a successful rat is one who is
reactive to changes, flees humans, finds rats of the opposite sex to
breed with, finds food and shelter in a complex environment and
avoids predators. In the domestic environment, the successful rat is
one who is passive to changes, tolerates humans, breeds willingly
with whatever members of the opposite sex are provided, tolerates
confinement, bright lights, poor hiding places, a simple environment,
and handling by human "predators."

Change their places, and many rats will have a lot of trouble
making the switch.

Natural selection is severe in the early generations after the
switch from the wild to captivity or from captivity to the wild.
Natural selection acts on populations: individuals that are poorly
adapted to their new environment die or fail to breed. Hence,
mortality and reproductive failure are high in wild rats moved to
captivity, and on domestic rats released in the wild. The survivors
are those that happen to have the traits that are compatible with
these new circumstances.

Here's an analogy. Consider a national sports team of your choice.
The players are extremely good at what they do: playing the sport.
Some are better than others, but all the members are very good. If
they weren't, they'd be eliminated from the team. Now transport them
to a new environment: put them all in a room and tell them that the
only way out is for them to demonstrate fluency in four languages.
All of a sudden, these sports stars find themselves poorly adapted to
their new environment. A small number of them may happen to speak
four languages, and they'll survive. The rest will fail.

The reverse situation is true too: place a group of linguists who
speak four languages in a room and tell them the only way out is to
demonstrate world-class mastery of a sport. A few of these linguists
may happen to have superb sporting ability, but most will fail.

It is the same with wild animals in the domestic environment or
domestic animals in the wild. The criteria for success have changed,
and most of the animals, well-adapted to their former environment,
find themselves poorly adapted to their new one. Many fail, some
survive.

The survivors form the basis of the next generation of animals in
the new environment. The second generation, and all subsequent
generations, will pass through the same seive, and only those who
survive and breed will pass on their traits to the next generations.
After generations, the new population may look and act quite
differently from the original population.

Natural selection isn't the only force acting on animals in new
environments. Relaxed selection (removal of natural selection on
certain traits) also plays a role in both environments, and
artificial selection (preferential breeding of animals with certain
traits by their human managers) plays a role in captivity.

Lastly, animals are not passive subjects of natural selection.
They can act on and respond to their environment, and many of them
may learn and adjust individually to their changed circumstances.

Wild rats in
captivity

Wild animals, including rats, are more active and reactive than
animals that have been domesticated for many generations.

Wild animals can be extremely stressed by the captive environment.
Compared to the wild, the captive environment is extremely confined,
provides few hiding places, is full of bright lights, and is
surrounded by human "predators" who approach and handle the animal.
These stressful conditions frequently lead to death or reproductive
failure in captive wild animals.

Mortality of wild animals in captivity

Mortality of wild animals in captivity can be severe during those
first few generations. For example, Blus (1971) established a
breeding colony of short-tailed shrews in captivity and found that
only 11% of his wild-caught shrews, and 9% of his captive-born
shrews, survived for 12 months. The mean age at death was only 5
months (reviewed in Price 2002).

Reproductive failure in captivity

Reproductive failure includes the failure to mate, failure to
produce normal-sized litters, and failure to rear young
successfully.

Reproductive failure is common in wild and early generation
animals in captivity. Only 49% of first-generation wild Norway rats
copulate successfully in captivity (Price 1980). Of rats who do give
birth, only 43% successfully raised some offspring to weaning age --
the rest were cannibalized or abandoned (Clark and Price 1981). Trut
(1999) found that only 14% of field-trapped Norway rats produced
offspring that survived to adulthood (see Price 2002).

Litters are generally smaller (averaging 6 offspring) in these
first-generation wild rats (Clark and Price 1981). In contrast, wild
rats in the wild, and domestic rats in captivity, produced
similarly-sized large litters, averaging about 10 offspring (Davis
1951, Boice 1972). It takes about 20 generations in captivity for rat
litter sizes to come back to normal (King 1929, 1939). Of course,
other changes are happening (e.g. reduction in brain size, Rohrs
1999, Kruska 1975a and b) that are not found in the wild stock, but I
won't go into those here.

Domestication: adapting to captivity

As a general rule, when a wild species is brought into captivity
it has an enormous adjustment to make to become successful in the
captive environment. This adjustment means the death or reproductive
failure of many, many individuals who just can't make the switch to
captivity.

Sometimes, the animals who die or fail to breed are the very
individuals who would be most successful in the wild: the flighty,
active individuals with low tolerance for humans. It is the passive,
less reactive, less fearful, calm individuals who survive and breed
best in captivity. These individuals form the foundation of the
domestic stock and their offspring carry on those traits (see Price
2002 for more on this).

This process is called natural selection in captivity, and
it is a huge watershed during those first few generations, with large
numbers of animals dying or failing to breed, regardless of the
actions and wishes of the human breeder. The resulting population of
animals, descended from the survivors, may be quite different from
the original wild stock, as these animals become adapted to their
new, captive environment. (Natural selection in captivity is a
problem for programs that breed endangered animals for release in the
wild, but that's another topic).

Natural selection in captivity is combined with artificial
selection, in which the human preferentially breeds some animals
over others in order to produce more animals with a desired trait in
the next generation.

Taming

Tamability is an important factor in domestication. Animals that
can tolerate the presence of and handling by humans are more likely
to survive and breed, due to both natural and artificial selection.
Natural selection plays a role because animals who cannot tolerate
human presence and handling may die or fail to breed. Artificial
selection also plays a role because humans prefer to work with
animals that are easy to capture and handle, and preferentially breed
animals with these traits.

Genetic and experiential factors influence tameness: individuals
inherit a greater or lesser capacity to be tamed (tamability), and
the experiences each animal has with humans determines the extent to
which that potential tameness is reached (Price 2002).

Tamability has a heritable component that responds well to
artificial selection. In rats, some coat colors are associated with
tameness: non-agouti (black) rats and hooded rats of wild stock are
more docile than their solid-colored agouti counterparts (more on
coat color and how it affects behavior)
(Keeler 1942, Cottle and Price 1987). Today, about 80% of domestic
laboratory rat strains are homozygous for the non-agouti allele
(Price 2002). This association between tameness and coat color has
been found in other species as well (e.g. deermice, Hayssen 1997;
foxes, Trut 1999).

Early handling by humans has a big effect on tameness as well.
Galef (1970) reared second and third generation wild rats (the direct
descendants of wild rats captured on Philadelphia wharfes) under
several conditions: with their wild mothers or with domestic mothers,
with wild or domestic litter mates, with minimal or maximal exposure
to humans, and with or without regular handling by humans (2 minutes
per day from age 10 days to 23 days). Only direct handling experience
was associated with ease of capture and handling. However, handling
did not affect the wild rats' timidity toward novel objects or
aggression toward other rats or mice. Handled wild rats were just as
timid as unhandled wild rats. Therefore, handling is quite context
specific: handling reduces aggression toward humans, but has little
effect on other behaviors typical of wild rats.

In a similar experiment, Hughes (1975) raised wild rats under
three conditions: (1) he let domestic mother rats rear one group of
wild babies, (2) he provided an enriched environment for a group of
post-weaning babies, and (3) he regularly handled a third group of
wild babies before they were weaned. He found that early handling had
a far greater effect on tameness than the other two conditions.
Handled rats showed less emotionality and were more like domestic
rats in their behavior. In contrast, being reared by a domestic
mother or playing in an enriched environment had only minimal effects
on tameness.

Evidence suggests that such early handling leads to major changes
in the neuroendorcine system (Denenberg et al. 1967). One
possible mechanism involves the responsivity of the adrenal glands.
Levine et al. (1967) handled 1-20 day old rats each day by
picking them up, placing them individually in a can partly filled
with shavings for three minutes, then replacing them in their home
cage. At age 80 days, the adrenal glands of the handled rats were
less responsive to stress when placed in an open arena for 3 minutes
(see also Levine 1968).

Not
surprisingly, there is less information on domestic animals adapting
to the wild (feralization) than on wild animals adapting to
captivity (domestication). We humans aren't often around to
observe feralization, but we are definitely around for
domestication.

Domestic animals released in the wild may have a number of
handicaps compared to their wild counterparts. They may lack some of
the structural, physiological and behavioral responses to
environmental stimuli that are normally acquired by their free-living
counterparts, usually early in life. These deficits may lead to
increased mortality of domestic rats released in the wild (see Price
2002 for more).

Physical condition: Living in the wild may require greater
physical condition than in captivity. The large size of some captive
animals may be due to better nutrition and lack of exercise. Such
large size may be a handicap in the wild if it impairs mobility and
agility (Price 2002).

Structural attributes: Many domestic rats are white or have
white patches on their fur. These patches may be conspicuous to
predators, leading to higher predation on animals with white fur. In
addition, albino animals have poor
vision, which may impair their chances of survival even more.

Behavioral responses: Released animals must find food and
shelter, develop anti-predator skills, interact appropriately with
conspecifics if they encounter them, and orient (disperse, navigate
etc.) in a complex environment (Price 2002). Deficits in behavior may
result in slower growth and higher mortality.

Success of domestic rats in the wild

Domestic rats are therefore at a disadvantage as compared to their
free-living counterparts. Reports of domestic rats establishing feral
populations in the wild are rare. Donaldson (1916) made four attempts
to feralize albino rats but was unsuccessful. King and Donaldson
(1929) tried five times to establish populations of feral albino
rats, at sites ranging from Massachusetts to an island group in the
Gulf of Mexico. All attempts failed.

However, in some cases, domestic rats may survive and establish
populations under natural or semi-natural conditions.

Domestic rats in semi-natural conditions

Albino rats in an outdoor pen in Missouri, USA: Boice
(1977) released ten domestic rats (5 males, 5 females) in a large,
fully-enclosed outdoor pen and studied them for two years. He found
that the rats constructed and lived in burrows that were
indistinguishable from wild rat burrows. They followed regular
pathways above ground like those seen in wild rats. The rats were
hardy throughout climactic extremes, surviving cold temperatures as
low as -30º C. The rats reproduced successfully, producing
litters mainly in the spring and fall. They established a stable
population that stayed around 50 rats, for a total of five
generations.

Behavior: Only females hoarded more than a few food pellets.
Pairs of males were aggressive toward each other (leaping,
kicking, sidling, and squealing) but serious fighting was not
observed. One rat was an outcast who never went into a burrow. He
huddled outside even during storms and survived under these
conditions for three months. Rats formed a deep rat-pile of as
many as 20 rats during cold weather, probably for heat
conservation. The rats followed pathways above ground and marked
them with urine. Feces were twice as likely to be above ground as
below it.

Burrows: All group members, male and female (except for old
males) burrowed. Rats of 90-110 days dug with the greatest
intensity. The burrows resembled those
of wild rats: They started with an entrance hold with dirt
scattered around half of it, they constructed a nest chamber at
the end of the first tunnel, they always constructed a bolt hole,
dug from within, from the second tunnel. The diameter, depth, and
length of the tunnels and the number of nest chambers were very
similar to those of wild rats.

Boice's experiment demonstrates that domestic rats can survive
harsh temperatures, construct shelter for themselves and breed
successfully under such conditions. They have not lost these
abilities even after hundreds of generations in captivity. Note,
however, that Boice's experiment was not a true release into the
wild: these rats were given food and water daily and were protected
from predators.

Albino and hooded rats in a farmyard in Oxfordshire, UK: In
another report, Manuel Berdoy released 75 albino and hooded
laboratory rats into an enclosure in a farmyard in Oxfordshire. The
rats found water, food, and shelter. They established paths through
their environment and dug burrows. They built social hierarchies and
bred successfully. Due to the enclosure, they were not confronted
with natural predators or with wild rats (documentary film Berdoy
2003, reported in Peplow
2004).

Beige dumbo rats under a chicken coop in New Orleans, US:
In the spring of 2004, a litter of six 5-6 week old beige dumbo rex
and standard-haired rats escaped from their cage, which was kept
outside on a porch in an urban neighborhood. The rats discovered a
chicken coop in the backyard and began living in the space underneath
the coop's floor, with a few deep tunnels. Food was provided to the
chickens every day so the rats had an easy food source. Attempts to
catch them were unsuccessful. As they did not harm the chickens they
were allowed to stay. When the floorboards to the coop were removed,
the rats moved to tunnels deeper underground. At least one litter was
produced under the coop, which was discovered when the coop floor was
taken up. The new litter born under the coop was brought back into
captivity, but the original escapees were not captured. No other
litters were known.

The rats had been extensively handled before their escape, and
were quite tame. After their escape, however, they became shy, though
not as shy as a wild rat. Most of the rats rarely emerged from the
coop, though a single bolder one made forays back onto the porch. The
original boldest rat, a female, probably died from an infected tail
injury that occurred during a failed capture attempt. She was
succeeded by a male. This male was quite unafraid and allowed a hand
to get almost, but not quite, within grabbing distance before
fleeing. The entire human family, including the dog, could stand on
the porch and watch him and he remained unperturbed, as long as
nobody moved quickly. If an attempt was made to catch him, he fled,
but was usually back the next night [L. J. pers. comm.
2005].

Note: Two additional adult rats that found their way out of
the cage eventually returned to captivity. One of these was the
escapees' mother, who had never been particularly tame. After a month
outdoors, she started coming to the porch again, and one day walked
up to one of the humans and sat on his shoe to receive a treat. She
also came through the open back door and into the house several
times. She was taken back into captivity and has since become quite
tame. The other adult escapee was an adult agouti female who was not
very tame. She escaped but refused to leave the vicinity of the porch
and after a day and a half managed to find her own way back into her
cage during cage cleaning [L. J. pers. comm.
2005].

Domestic rats in the wild

Albino rat colony in Montana, USA:Minckler
and Pease (1938) mention a colony of albino rats living in a landfill
in Montana, which numbered about 2,000 rats in 1937. The exact source
of these rats was unknown, but they were presumed to have been
released by students from the local university. Abundant food, water,
refuge and few predators created a sheltered environment in which an
albino colony could survive, even in harsh winters with temperatures
as low as -25 F.

Interestingly, these albino rats traveled on paths through the
refuse heap, and they never left these paths, even to try a fragrant
food source just a few inches to one side of the path. Apparently,
these rats had to come directly on the food, almost touching it,
before they responded to it.

Diseases and parasites: several of Minckler and
Pease's rats had liver cysts, and a few had "crusted eyes" and
showed symptoms of the common "mouse pneumonia" [by this, I
assume the authors mean the eyes were crusted with porphyrin and
the rats had symptoms of mycoplasma pulmonis -- A.]. Many rats
had hair loss, possibly due to parasites or a dietary deficiency.
A large number, especially among the younger rats, were affected
with rickets.

Albino and hooded rats on Lanai, Hawaii, USA:Svihla
(1936) reported albino and "spotted" (white belly and sides --
probably hooded) Norway rats living in fields under natural
conditions on the island of Lanai, which is an island southeast of
Hawaii. This island environment is sheltered: food is abundant, there
is no competition for habitat, and there are few predators (no
mongooses, few feral cats and native owls). The rats were presumed to
be the descendants of escaped pet rats belonging to Filipino
plantation hands. The escaped rats interbred and became common in the
pineapple fields, houses, and buildings of Lanai City.

Interactions
between wild and domestic rats

Very few studies place domestic and wild rats together, so there
is little information on how wild and domestic rats might get
along.

Note, however, that resident rats usually attack intruders in
their colony -- domestic rats attack domestic intruders, and wild
rats attack wild intruders. Studies of rat aggression against
intruders are very common. To read more about them, visit the
aggression page. Therefore, a reasonable
prediction about wild and domestic rat interactions is that resident
wild rats would attack a domestic rat intruder, and domestic rats
would attack a wild rat intruder.

How are domestic rats received in wild colonies?

To my knowledge, no studies have examined how wild rat colonies
receive a domestic intruder in the wild.

However, one study has examined how colonies of wild rats in
captivity received a domestic intruder. The wild rats attacked the
domestic intruder intensely. Most attacks were performed by the
dominant male, who lunged and leaped at, sidled, chased, and bit the
intruder. The intruder fled, froze, and spent time on his back. The
wild dominant rat inflicted about 10 bites on the intruder in 10
minutes, and most of the bites were to the intruder's back. For
comparison, wild intruders introducted to wild colonies in captivity
receive about 6.9 bites in 10 minutes (Takahashi and Blanchard
1982).

How do domestic rats receive wild rats?

When a wild Norway rat was placed in a colony of domestic rats in
captivity, the level of dominant male rat attack on the intruder was
low. This may have been because the wild intruder displayed intense
defensive behavior, and the wild intruder was also much faster than
the domestic rats. Therefore, the dominant male chased him but rarely
managed to catch him. In addition to chasing, the domestic rat
sidled, and when possible, bit the wild intruder. The intruder
displayed defensive behavior: he fled, froze, boxed, and spent time
on his back. The dominant rats inflicted about 1 bite on the wild
intruders in 10 minutes, and most of the bites were to the intruder's
back. For comparison, domestic intruders introduced to domestic
colonies receive about 5.6 bites in 10 minutes (Takahashi and
Blanchard 1982).

Conclusion

Wild-domestic rat interactions are similar to wild-wild and
domestic-domestic interactions. Regardless of whether the rats are
wild or domestic, resident rats, especially the dominant males, tend
to attack intruders. The two asymmetries uncovered between wild and
domestic rat are that wild rats display a lunging or leaping attack
rarely seen in domestic males, and wild rats are faster than domestic
rats. This speed difference means that wild rats may have an
advantage in wild-domestic interactions: when the wild rat is the
resident, he can press a more effective attack, and when the wild rat
is the intruder, he can elude attack more easily.